An armor-piercing shell,[a]AP for short, is a type of ammunition designed to penetrate armor. From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions. AP rounds smaller than 20 mm are typically known as "armor-piercing ammunition", and are intended for lightly-armored targets such as body armor, bulletproof glass and light armored vehicles. The classic AP shell is now seldom used in naval warfare, as modern warships have little or no armor protection, and newer technologies have displaced the classic AP design in the anti-tank role.

An armor-piercing shell must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened body with a specially hardened and shaped nose. One common addition to later AP shells is the use of a softer ring or cap of metal on the nose known as a penetrating cap, which both lowers the initial shock of impact to prevent the rigid shell from shattering, as well as aiding the contact between the target armor and the nose of the penetrator to prevent the shell from bouncing off in glancing shots. Ideally, these caps have a blunt profile, which led to the use of a thin aerodynamic cap to improve long-range ballistics. AP shells may contain little or no explosive, in this case known as a "bursting charge". Some smaller-caliber AP shells have an inert filling or incendiary charge in place of the bursting charge.

As tank armor improved during World War II, AP designs were introduced that used a smaller penetrating body within a larger shell. These lightweight shells fired at very high muzzle velocity and retained that speed and the associated penetrating power over longer distances. In modern designs the penetrator no longer looks like a classic artillery shell design, but is instead a long rod of dense material like tungsten or depleted uranium (DU) that further improves the terminal ballistics. Whether these designs are considered to be AP rounds depends on the definition and may be included or excluded from reference to reference.

The first solution to this problem was effected by Major Sir W. Palliser, who, with the Palliser shot, invented a method of hardening the head of the pointed cast-iron shot.[1][b] By casting the projectile point downwards and forming the head in an iron mold, the hot metal was suddenly chilled and became intensely hard (resistant to deformation through a Martensite phase transformation), while the remainder of the mold, being formed of sand, allowed the metal to cool slowly and the body of the shot to be made tough[1] (resistant to shattering).

These chilled iron shots proved very effective against wrought iron armor but were not serviceable against compound and steel armor,[1] which was first introduced in the 1880s. A new departure, therefore, had to be made, and forged steel rounds with points hardened by water took the place of the Palliser shot. At first, these forged-steel rounds were made of ordinary carbon steel, but as armor improved in quality, the projectiles followed suit.[1]

During the 1890s and subsequently, cemented steel armor became commonplace, initially only on the thicker armor of warships. To combat this, the projectile was formed of steel—forged or cast—containing both nickel and chromium. Another change was the introduction of a soft metal cap over the point of the shell – so called "Makarov tips" invented by Russian admiral Stepan Makarov. This "cap" increased penetration by cushioning some of the impact shock and preventing the armor-piercing point from being damaged before it struck the armor face, or the body of the shell from shattering. It could also help penetration from an oblique angle by keeping the point from deflecting away from the armor face.

AP shells containing an explosive filling were initially distinguished from their non-HE counterparts by being called a "shell" as opposed to "shot", mostly used in British terminology with the invention of the first shell of this type, the Palliser shell with 1.5% HE in the shells in 1877. By the time of the Second World War, AP shells with a bursting charge were sometimes distinguished by appending the suffix "HE". At the beginning of the war, APHE was common in anti-tank shells of 75 mm caliber and larger due to the similarity with the much larger naval armour piercing shells already in common use. As the war progressed, ordnance design evolved so that the bursting charges in APHE became ever smaller to non-existent, especially in smaller caliber shells, e.g. Panzergranate 39 with only 0.2% HE filling. Modern full caliber armor-piercing shells as dedicated anti-tank projectiles are no longer the primary method of conducting anti-tank warfare, but are still in use in over 50mm caliber artillery, however the tendency is to use semi-armor-piercing high-explosive (SAPHE) shells, which have less anti-armor capability, but far greater anti-materiel/personnel effects. The modern SAPHE projectiles still have a ballistic cap, hardened body and base fuze, but tend to have a far thinner body material and much higher explosive content (4–15%). Common abbreviations for modern AP and SAP shells are: HEI(BF), SAPHE, SAPHEI, and SAPHEI-T. The primary shell types for modern anti-tank warfare are kinetic energy penetrators, such as APDS.

Shot and shell used prior to and during World War I were generally cast from special chromium (stainless) steel that was melted in pots. They were forged into shape afterward and then thoroughly annealed, the core bored at the rear and the exterior turned up in a lathe.[1] The projectiles were finished in a similar manner to others described above. The final, or tempering treatment, which gave the required hardness/toughness profile (differential hardening) to the projectile body, was a closely guarded secret.[1]

The rear cavity of these projectiles was capable of receiving a small bursting charge of about 2% of the weight of the complete projectile; when this is used, the projectile is called a shell, not a shot. The HE filling of the shell, whether fuzed or unfuzed, had a tendency to explode on striking armor in excess of its ability to perforate.[1]

During World War II, projectiles used highly alloyed steels containing nickel-chromium-molybdenum, although in Germany, this had to be changed to a silicon-manganese-chromium-based alloy when those grades became scarce. The latter alloy, although able to be hardened to the same level, was more brittle and had a tendency to shatter on striking highly sloped armor. The shattered shot lowered penetration, or resulted in total penetration failure; for armor-piercing high-explosive (APHE) projectiles, this could result in premature detonation of the HE filling. Highly advanced and precise methods of differentially hardening the projectile were developed during this period, especially by the German armament industry. The resulting projectiles gradually change from high hardness (low toughness) at the head to high toughness (low hardness) at the rear and were much less likely to fail on impact.

APHE shells for tank guns, although used by most forces of this period, were not used by the British. The only British APHE projectile for tank use in this period was the Shell AP, Mk1 for the 2 pdr anti-tank gun and this was dropped as it was found that the fuze tended to separate from the body during penetration. Even when the fuze didn’t separate and the system functioned correctly, damage to the interior was little different from the solid shot, and so did not warrant the additional time and cost of producing a shell version. They had been using APHE since the invention of the 1.5% HE Palliser shell in the 1870s and 1880s, and understood the tradeoffs between reliability, damage, HE %, and penetration, and deemed reliability and penetration to be most important for tank use. Naval APHE projectiles of this period, being much larger used a bursting charge of about 1–3% of the weight of the complete projectile,[1] but in anti-tank use, the much smaller and higher velocity shells used only about 0.5% e.g. Panzergranate 39 with only 0.2% HE filling. This was due to much higher armor penetration requirements for the size of shell (e.g. over 2.5 times caliber in anti-tank use compared to below 1 times caliber for naval warfare). Therefore, in most APHE shells put to anti-tank use the aim of the bursting charge was to aid the number of fragments produced by the shell after armor penetration, the energy of the fragments coming from the speed of the shell after being fired from a high velocity anti-tank gun, as opposed to its bursting charge. There were some notable exceptions to this, with naval caliber shells put to use as anti-concrete and anti-armor shells, albeit with a much reduced armor penetrating ability. The filling was detonated by a rear-mounted delay fuze. The explosive used in APHE projectiles needs to be highly insensitive to shock to prevent premature detonation. The US forces normally used the explosive Explosive D, otherwise known as ammonium picrate, for this purpose. Other combatant forces of the period used various explosives, suitably desensitized (usually by the use of waxes mixed with the explosive).

HEAT shells are a type of shaped charge used to defeat armoured vehicles. They are extremely efficient at defeating plain steel armour but less so against later composite and reactive armour. The effectiveness of the shell is independent of its velocity, and hence the range: it is as effective at 1000 metres as at 100 metres. This is because HEAT shells don’t lose penetration over distance, in fact the speed can even be zero in the case where a soldier simply places a magnetic mine onto a tank's armour plate. A HEAT charge is most effective when detonated at a certain, optimal, distance in front of the target and HEAT shells are usually distinguished by a long, thin nose probe sticking out in front of the rest of the shell and detonating it at the correct distance, e.g., PIAT bomb. HEAT shells are less effective if spun (i.e., fired from a rifled gun).

HEAT shells were developed during the Second World War as a munition made of an explosive shaped charge that uses the Munroe effect to create a very high-velocity partial stream of metal in a state of superplasticity, and used to penetrate solid vehicle armour. HEAT rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. A single infantryman could effectively destroy any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. During World War II, weapons using HEAT warheads were known as having a hollow charge or shape charge warhead.[2]

Claims for priority of invention are difficult to resolve due to subsequent historic interpretations, secrecy, espionage, and international commercial interest.[3]Shaped charge warheads were promoted internationally by the Swiss inventor Henry Mohaupt, who exhibited the weapon before the second World War. Prior to 1939 Mohaupt demonstrated his invention to British and French ordnance authorities. During the war, the French communicated Henry Mohaupt's technology to the U.S. Ordnance Department, who invited him to the USA, where he worked as a consultant on the Bazooka project. By mid-1940, Germany had introduced the first HEAT round to be fired by a gun, the 7.5 cm fired by the Kw.K.37 L/24 of the Panzer IV tank and the Stug III self-propelled gun (7.5 cm Gr.38 Hl/A, later editions B and C). In mid-1941, Germany started the production of HEAT rifle-grenades, first issued to paratroopers and by 1942 to the regular army units. In 1943, the Püppchen, Panzerschreck and Panzerfaust were introduced. The Panzerfaust and Panzerschreck or 'tank terror' gave the German infantryman the ability to destroy any tank on the battlefield from 50 – 150 m with relative ease of use and training (unlike the UK PIAT).

The first British HEAT weapon to be developed and issued was a rifle grenade using a 2 1/2 inch cup launcher on the end of the barrel; the British No. 68 AT grenade issued to the British army in 1940. By 1943, the PIAT was developed; a combination of a HEAT warhead and a spigot mortar delivery system. While cumbersome, the weapon at last allowed British infantry to engage armour at range; the earlier magnetic hand-mines and grenades required them to approach suicidally close.[4] During World War II, the British referred to the Munroe effect as the cavity effect on explosives.[2]

High-explosive, squash-head (HESH) is another anti-tank shell based on the use of explosive. A thin-walled shell case contains a large charge of a plastic explosive. On impact the explosive flattens, without detonating, against the face of the armour, and is then detonated by a fuze in the base of the shell. Energy is transferred through the armour plate: when the compressive shock reflects off the air/metal interface on the inner face of the armour, it is transformed into a tension wave which spalls a "scab" of metal off into the tank damaging the equipment and crew without actually penetrating the armour.

HESH is defeated by spaced armour, so long as the plates are individually able to withstand the explosion. It is still considered useful as not all vehicles are equipped with spaced armour, and it is also the most effective munition for demolishing brick and concrete. HESH shells, unlike HEAT shells, can be fired from rifled guns as they are unaffected by spin. In American usage it is known as high-explosive plastic (HEP).

The high-explosive squash head (HESH) was developed by Charles Dennistoun Burney in the 1940s for the British war effort, originally as an anti-fortification "wallbuster" munition for use against concrete. HESH rounds were thin metal shells filled with plastic explosive and a delayed-action base fuze. The plastic explosive is "squashed" against the surface of the target on impact and spreads out to form a disc or "pat" of explosive. The base fuze detonates the explosive milliseconds later, creating a shock wave that, owing to its large surface area and direct contact with the target, is transmitted through the material. At the point where the compression and tension waves intersect a high-stress zone is created in the metal, causing pieces of steel to be projected off the interior wall at high velocity. This fragmentation by blast wave is known as spalling, with the fragments themselves known as spall. Unlike high-explosive anti-tank (HEAT) rounds, which are shaped charge ammunition, HESH shells are not specifically designed to perforate the armour of main battle tanks. HESH shells rely instead on the transmission of the shock wave through the solid steel armour.

HESH was found to be surprisingly effective against metallic armour as well, although the British already had effective weapons using HEAT, such as the PIAT. HESH was for some time a competitor to the more common HEAT round, again in combination with recoilless rifles as infantry weapons and was effective against tanks such as the T-55 and T-62.

Armor-piercing solid shot for cannons may be simple, or composite, solid projectiles but tend to also combine some form of incendiary capability with that of armor-penetration. The incendiary compound is normally contained between the cap and penetrating nose, within a hollow at the rear, or a combination of both. If the projectile also uses a tracer, the rear cavity is often used to house the tracer compound. For larger-caliber projectiles, the tracer may instead be contained within an extension of the rear sealing plug. Common abbreviations for solid (non-composite/hardcore) cannon-fired shot are; AP, AP-T, API and API-T; where "T" stands for "tracer" and "I" for "incendiary". More complex, composite projectiles containing explosives and other ballistic devices tend to be referred to as armor-piercing shells.

Early WWII-era uncapped (AP) armor-piercing projectiles fired from high-velocity guns were able to penetrate about twice their caliber at close range (100 m). At longer ranges (500-1,000 m), this dropped 1.5–1.1 calibers due to the poor ballistic shape and higher drag of the smaller-diameter early projectiles. In January 1942 a process was developed by Arthur E. Schnell [5] for 20mm and 37mm Armor Piercing rounds to press bar steel under 500 tons of pressure that made more even "flow-lines" on the tapered nose of the projectile which allowed the shell to follow a more direct nose first path to the armor target. Later in the conflict, APCBC fired at close range (100 m) from large-caliber, high-velocity guns (75–128 mm) were able to penetrate a much greater thickness of armor in relation to their caliber (2.5 times) and also a greater thickness (2–1.75 times) at longer ranges (1,500–2,000 m).

In an effort to gain better aerodynamics APC rounds were given a ballistic cap to improve muzzle velocity and reduce drag. The hollow ballistic cap would break away when the projectile hit the target. These rounds were classified as (APBC) or armor-piercing ballistic capped rounds.

Due to the increase in armor thickness during WWII, the projectiles’ size and impact velocity had to be increased to ensure perforation. At these higher velocities, the hardened tip of the shot or shell has to be protected from the initial impact shock, or risk shattering. To raise the impact velocity and stop the shattering, they were initially fitted with soft steel penetrating caps. The resulting rounds were classified as (APC) or armor piercing capped.

Since the best performance penetrating caps were not very aerodynamic, an additional ballistic cap was later fitted to reduce drag. The resulting rounds were classified as (APCBC) or armor-piercing capped ballistic capped. The hollow ballistic cap gave the rounds a sharper point which reduced drag and broke away on impact.[6]

An important armor-piercing development was the (APDS) or the armor-piercing discarding sabot. An early version was developed by engineers working for the French Edgar Brandt company, and was fielded in two calibers (75 mm/57 mm for the Mle1897/33 75 mm anti-tank cannon, 37 mm/25 mm for several 37 mm gun types) just before the French-German armistice of 1940.[7] The Edgar Brandt engineers, having been evacuated to the United Kingdom, joined ongoing APDS development efforts there, culminating in significant improvements to the concept and its realization. The APDS projectile type was further developed in the United Kingdom between 1941-1944 by L. Permutter and S. W. Coppock, two designers with the Armaments Research Department. In mid-1944 the APDS projectile was first introduced into service for the UK’s QF 6 pdranti-tank gun and later in September 1944 for the 17 pdr anti-tank gun.[8] The idea was to use a stronger penetrator material to allow increased impact velocity and armor penetration.

The armor-piercing concept calls for more penetration capability than the target's armor thickness. Generally, the penetration capability of an armor-piercing round increases with the projectile's kinetic energy and also with concentration of that energy in a small area. Thus, an efficient means of achieving increased penetrating power is increased velocity for the projectile. However, projectile impact against armor at higher velocity causes greater levels of shock. Materials have characteristic maximum levels of shock capacity, beyond which they may shatter, or otherwise disintegrate. At relatively high impact velocities, steel is no longer an adequate material for armor-piercing rounds. Tungsten and tungsten alloys are suitable for use in even higher-velocity armor-piercing rounds, due to their very high shock tolerance and shatter resistance, and to their high melting and boiling temperatures. They also have very high density. Energy is concentrated by using a reduced-diameter tungsten shot, surrounded by a lightweight outer carrier, the sabot (a French word for a wooden shoe). This combination allows the firing of a smaller diameter (thus lower mass/aerodynamic resistance/penetration resistance) projectile with a larger area of expanding-propellant "push", thus a greater propelling force and resulting kinetic energy. Once outside the barrel, the sabot is stripped off by a combination of centrifugal force and aerodynamic force, giving the shot low drag in flight. For a given caliber, the use of APDS ammunition can effectively double the anti-tank performance of a gun.

An armor-piercing, fin-stabilized, discarding sabot (APFSDS) projectile uses the sabot principle with fin (drag) stabilization. A long, thin sub-projectile has increased sectional density and thus penetration potential. However, once a projectile has a length-to-diameter ratio greater than 10[citation needed] (less for higher density projectiles),[citation needed] spin stabilization becomes ineffective. Instead, aerodynamic lift stabilization is used, by means of fins attached to the base of the sub-projectile, making it look like a large metal arrow.

Large caliber APFSDS projectiles are usually fired from smooth-bore (unrifled) barrels, though they can be and often are fired from rifled guns. This is especially true when fired from small to medium caliber weapon systems. APFSDS projectiles are usually made from high-density metal alloys, such as tungsten heavy alloys (WHA) or depleted uranium (DU); maraging steel was used for some early Soviet projectiles. DU alloys are cheaper and have better penetration than others, as they are denser and self-sharpening. Uranium is also pyrophoric and may become opportunistically incendiary, especially as the round shears past the armor exposing non-oxidized metal, but both the metal's fragments and dust contaminate the battlefield with toxic hazards. The less toxic WHAs are preferred in most countries except the USA, and Russia.[citation needed]

Armor-piercing, composite rigid (APCR) is a British term; the US term for the design is high-velocity armor-piercing (HVAP) and the German term is Hartkernmunition. The APCR projectile has a core of a high-density hard material, such as tungsten carbide, surrounded by a full-bore shell of a lighter material (e.g., an aluminium alloy). However, the low sectional density of the APCR resulted in high aerodynamic drag. Tungsten compounds such as tungsten carbide were used in small quantities of inhomogeneous and discarded sabot shot, but that element was in short supply in most places. Most APCR projectiles are shaped like the standard APCBC shot (although some of the German Pzgr. 40 and some Soviet designs resemble a stubby arrow), but the projectile is lighter: up to half the weight of a standard AP shot of the same caliber. The lighter weight allows a higher velocity. The kinetic energy of the shot is concentrated in the core and hence on a smaller impact area, improving the penetration of the target armor. To prevent shattering on impact, a shock-buffering cap is placed between the core and the outer ballistic shell as with APC rounds. However, because the shot is lighter but still the same overall size it has poorer ballistic qualities, and loses velocity and accuracy at longer ranges. The British devised a way for the outer sheath to be discarded after leaving the bore. The name given to the discarded outer sheath was the sabot (a French word for a wooden shoe, also used to describe the standardized wood or paper-mache wadding around round shot in a smooth bore cannon). The APCR was superseded by the APDS, which dispensed with the outer light alloy shell once the shot had left the barrel. The concept of a heavy, small-diameter penetrator encased in light metal would later be employed in small-arms armour-piercing incendiary and HEIAP rounds.

Armour-piercing, composite non-rigid (APCNR) is the British term and known by the Germans as Gerlich principle weapons, but today the more commonly used terms are squeeze-bore and tapered bore. These shells are based on the same projectile design as the APCR - a high density core within a shell of soft iron or other alloy - but it is fired by a gun with a tapered barrel, either a taper in a fixed barrel or a final added section. The projectile is initially full-bore, but the outer shell is deformed as it passes through the taper. Flanges or studs are swaged down in the tapered section, so that as it leaves the muzzle the projectile has a smaller overall cross-section.[6] This gives it better flight characteristics with a higher sectional density, and the projectile retains velocity better at longer ranges than an undeformed shell of the same weight. As with the APCR, the kinetic energy of the round is concentrated at the core on impact. The initial velocity of the round is greatly increased by the decrease of barrel cross-sectional area toward the muzzle, resulting in a commensurate increase in velocity of the expanding propellant gases.

In contrast, the British used the Littlejohn squeeze-bore adaptor, which could be attached or removed as necessary. The adaptor extended the usefulness of armoured cars and light tanks, which could not fit any gun larger than the QF 2 pdr. Although a full range of shells and shot could be used, changing the adaptor in the heat of battle was highly impractical.

There are some significant drawbacks that are inherent with weapons designed to fire APCNR rounds. The first is that designing and producing tapered bore guns requires both an advanced level of technology and high quality standards in manufacturing the gun barrels, resulting in a higher cost per unit. The second is that by tapering the bore to increase the velocity of the round subjects it to increased wear from having to deform the projectile during firing, shortening the barrel life of the weapon.

The APCNR was superseded by the APDS design which was compatible with non-tapered barrels.

Armor-piercing rifle and pistol cartridges are usually built around a penetrator of hardened steel, tungsten, or tungsten carbide, and such cartridges are often called 'hard-core bullets'. Aircraft and tank rounds sometimes use a core of depleted uranium. The penetrator is a pointed mass of high-density material that is designed to retain its shape and carry the maximum possible amount of energy as deeply as possible into the target. Depleted-uranium penetrators have the advantage of being pyrophoric and self-sharpening on impact, resulting in intense heat and energy focused on a minimal area of the target's armor. Some rounds also use explosive or incendiary tips to aid in the penetration of thicker armor. High Explosive Incendiary/Armor Piercing Ammunition combines a tungsten carbide penetrator with an incendiary and explosive tip.

Rifle armor-piercing ammunition generally carries its hardened penetrator within a copper or cupronickel jacket, similar to the jacket which would surround lead in a conventional projectile. Upon impact on a hard target, the copper case is destroyed, but the penetrator continues its motion and penetrates the target. Armor-piercing ammunition for pistols has also been developed and uses a design similar to the rifle ammunition. Some small ammunition, such as the FN 5.7mm round, is inherently capable at piercing armor, being of a small caliber and very high velocity.

The entire projectile is not normally made of the same material as the penetrator because the physical characteristics that make a good penetrator (i.e. extremely tough, hard metal) make the material equally harmful to the barrel of the gun firing the cartridge.

Most modern active protection systems (APS) are unlikely to be able to defeat full-caliber AP rounds fired from a large-caliber anti-tank gun, because of the high mass of the shot, its rigidity, short overall length, and thick body. The APS uses fragmentation warheads or projected plates, and both are designed to defeat the two most common anti-armor projectiles in use today: HEAT and kinetic energy penetrator. The defeat of HEAT projectiles is accomplished through damage/detonation of the HEAT's explosive filling or damage to the shaped charge liner or fuzing system, and defeat of kinetic energy projectiles is accomplished by inducing yaw/pitch or fracturing of the rod.

^"Shot" in this sense is a solid-metal artillery projectile similar to a "shell", but without any explosive charge. It is also used to describe other non-explosive artillery projectiles such as case shot or grape shot